Many proteins of higher organisms are normally glycosylated. The structure of the protein bound carbohydrate has been found to be a variously branched oligosaccharide composed of diverse monosaccharide subunits. It is generally supposed that the structure of the oligosaccharide is determined in part by the specificity of cellular processing enzymes (glycosidases and glycosyl transferases) and in part by the primary structure of the substrate (target) protein. However, the molecular basis by which structure is specified is largely unknown. There are several important technical advantages in using the immunoglobulin Mu heavy chain as a model system for glycosylation. Of particular relevance for this proposal is a collection of mutant hybridoma cell lines which we recently isolated in which the Mu heavy chain is abnormally glycosylated. I propose here to analyze the genetic alteration of these mutants in order to define the molecular features which specify the normal glycosylation pattern.